This invention relates generally to display device drive circuits and particularly to a drive circuit for a liquid crystal display.
Many display devices, such as liquid crystal displays, are composed of a matrix of active elements, or pixels, arranged vertically in columns and horizontally in rows. The data to be displayed are applied as drive voltages to data lines which are individually associated with each column of active elements. The rows of active elements are sequentially scanned and the individual active elements within the activated row are illuminated to various grey scale levels in accordance with the levels of the drive voltage applied to the various columns.
Accurate analog drive circuits are needed to drive color liquid crystal television displays, and high quality liquid crystal data displays. The use of discrete analog drive circuits for each of the data lines, which typically number 1440, is impractical because of cost, power and noise considerations. For this reason, time domain averaging and transmission gate multiplexing have been tried. In time domain averaging, grey scales are achieved by applying the full voltage to the liquid crystals for only a fraction of the full display time, so that when averaged over the full display time the RMS level is only a fraction of the fully on value of the liquid crystals. Such techniques produce only very crude three or four bits, i.e. eight to sixteen levels, of grey scale and significantly increase flicker of the display. Attempts have also been made to multiplex the analog signal directly into the array via transmission gates. However, only a small fraction of the line time is available to charge the data line capacitance, which typically is 20 to 100 picofarads, depending upon the size of the array. The speed of performance needed for the transmission gate increases in proportion to the number of display elements in the array and, therefore, multiplexing is not satisfactory for scaling arrays of more than 16,000 pixels, even if very high mobility polysilicon transmission gates are used. The display of color television on a liquid crystal requires an array having 250,000 to 750,000 pixels. Accordingly, multiplexing is unsatisfactory for such displays. Also, multiplexing would leave unequal time for pixels along the same word line to equalize with the data line, and would result in unequal grey scale smearing along the data lines. This undesirable result occurs because the columns are sequentially scanned and the later scanned columns have less time to settle than the earlier scanned columns. For these reasons, there is a need for an accurate analog drive circuit for liquid crystal displays including the large number of active elements needed for a color television display, or for a high quality data display. The present invention fulfills this need.